1. Field of the Invention
The present invention relates to a process for producing a defect-free photoelectric conversion device. More particularly, the present invention relates to an improved process for producing a defect-free photoelectric conversion device such as a solar cell, photosensor, etc. while repairing a defective portion such as pinhole, hillock, or the like, which occurred during the preparation thereof.
2. Related Background Art
In recent years, various studies have been made of large area photoelectric conversion devices such as solar cells, photosensors, electrophotographic photosensitive devices, flat panel displays, etc. Attention has been focused on non-single crystal semiconductors such as amorphous silicon semiconductors to constitute these large area semiconductor devices mainly because of their reasonable production cost.
For instance, there is known a pin junction type amorphous silicon solar cell as an example of such non-single crystal semiconductor device. In this solar cell, photocarriers are created in its semiconductor layer comprising an amorphous silicon semiconductor thin film when light impinges the solar cell. The photocarriers migrate to a transparent electrode comprising a transparent conductive thin film situated on the side through which light is impinged and also to a conductive substrate situated opposite the transparent electrode by the action of an internal electric field, to thereby provide a photoelectromotive force.
The conductive thin film forming the transparent electrode and the semiconductor thin film forming the semiconductor layer may be formed in a vacuum chamber in accordance with a plasma CVD method, a photo CVD method, a thermal CVD method, a vacuum evaporation method, or a sputtering method.
In the preparation of such amorphous silicon solar cell, there is often found a problem that a short circuit occurs between the transparent electrode and the conductive substrate due to pinholes formed at parts of the semiconductor layer.
There are various causes for such pinholes to be formed. For instance, in one case, since the transparent electrode is usually some hundreds of angstroms in thickness and the semiconductor layer is usually about 0.005 to some tens of .mu.m thick, minute dust particles (some micrometers to some tens of micrometers in size) are deposited on the surface of the conductive substrate or they are deposited on or contaminate the semiconductor layer during film formation, whereby pinholes are formed in the semiconductor layer.
In another case, pinholes are formed when part of the semiconductor layer is lost due to its internal stress or its insufficient adhesion with the transparent electrode. In this case, the transparent electrode situated on the side through which light is impinged and the conductive substrate situated opposite said electrode are connected with each other through said region to be in an electrically short-circuited state, and because of this, the result has extremely poor characteristics as a semiconductor device.
The occurrence of pinholes causing a short circuit is a serious problem particularly in the case of a large area photoelectric conversion device such as a solar cell, electrophotographic photosensitive device, etc. In any case, it is extremely difficult to obtain a large area photoelectric conversion device completely free of a short-circuited region even under a clean environment substantially free of minute dust particles.
In order to solve the above problem relative to occurrence of pinholes causing a short circuit, Japanese Patent Publication 62(1987)-53958 (hereinafter referred to as Literature 1) proposes a method of making the inside of each of the pinholes formed in the thin film semiconductor layer of a photosemiconductor to be in an electrically insulating state by perforating the electrode layer at the pinholes and communicating the pinholes of the thin film semiconductor layer with the pinholes of the electrode layer. Similarly, Japanese Patent Publication 62(1987)-59901 (hereinafter referred to as Literature 2) proposes a method of obviating pinholes formed in the thin film semiconductor layer of a semiconductor device by fusing the peripheries of the pinholes with radiation of energy beam.
FIGS. 10(a) and 10(b) are schematic views for explaining the method according to Literature 1.
In FIGS. 10(a) and 10(b), reference numeral 1 indicates a translucent substrate, reference numeral 2 indicates a translucent electrode layer, reference numeral 3 indicates a semiconductor layer comprising a thin semiconductor film, reference numeral 4 indicates a back electrode layer, reference numeral 5 indicates a pinhole in a short-circuited state, reference numeral 6 indicates a pinhole provided at the back electrode layer, reference numeral 7 indicates a laser beam, and reference numeral 8 indicates another laser beam.
The method according to Literature 1 will be explained with reference to FIGS. 10(a) and 10(b). That is, after a plurality of semiconductor devices have been prepared, the semiconductor devices which are defective because of short circuits are sorted out. As for each of those defective semiconductor devices, beam scanning is performed while irradiating laser beam 7 through the other principal face of the translucent substrate as shown in FIG. 10(a). When the short circuit current is measured for the semiconductor device at the time of performing the beam scanning, short circuit current does not flow when the laser beam 7 is irradiated to the portion where a pinhole 5 in a short-circuited state is present. On the other hand, upon irradiating the laser beam 7 to other portions where such short-circuited state is not present, hole-electron pairs are created and they migrate in the semiconductor layer 3, whereby current flows. In view of this, the position where a pinhole 5 is present can be found for the semiconductor device by performing beam scanning using the laser beam 7.
As for the portion of the semiconductor layer where a pinhole 5 is present, the laser beam outputted from a YAG pulsed laser of 5.times.10.sup.6 W/cm.sup.2 peak output power is radiated through the back electrode layer 4 as shown by arrow 8 to thereby remove a short-circuited region comprising the material of the back electrode layer 4 which extends to the inside of the pinhole 5. Particularly, as shown in FIG. 10(b), a pinhole 6 is formed at the back electrode layer 4 such that it is coaxially in communication with the pinhole 5, whereby the inside of the pinhole 5 and that of the pinhole 6 are in an electrically insulating state.
FIGS. 10(c), 10(d), and 10(e) are schematic views for explaining the method according to Literature 2.
In FIGS. 10(c), 10(d), and 10(e), reference numeral 1 indicates a translucent substrate, reference numeral 2 indicates a translucent electrode layer, reference numeral 3 indicates a semiconductor layer comprising a thin semiconductor film, reference numeral 4 indicates a back electrode layer, reference numeral 5 indicates a pinhole in a short-circuited state, reference numeral reference 7 indicates a laser beam, and reference numeral 9 indicates a photosensor.
Translucent electrode 2 is formed on a translucent substrate 1 and then, a thin film semiconductor layer 3 is formed on the translucent electrode 2. As for the device thus obtained, beam scanning is performed on the semiconductor layer 3 by irradiating laser beam 7 from an Ar gas laser of an extremely low output power through the rear side of the semiconductor layer 3 and moving a photosensor 9 arranged on the side of the translucent substrate 1 opposite the Ar gas laser, in synchronism with the scanning of the laser beam 7, as shown in FIG. 10(c), to thereby determine whether or not the semiconductor layer 3 contains a pinhole 5. In this case, if such pinhole 5 is not present at the portion of the semiconductor layer 3 where the laser beam 7 is irradiated, the laser beam 7 is absorbed by the semiconductor 3 and does not reach the photosensor 9. On the other hand, if such pinhole 5 is present at the portion of the semiconductor layer 3 where the laser beam 7 is irradiated, the laser beam 7 reaches the photosensor 9, and based on a signal outputted from the photosensor 9 at that time, the position where the pinhole 5 is present is detected.
When the presence of the pinhole 5 is optically detected as above described, a laser beam of about 2 to 3 W/cm.sup.2 in power outputted from, for example, an Ar gas laser of 514.5 nm wavelength instead of the laser beam 7 is irradiated to the portion where the pinhole 5 is present, to thereby fuse the peripheries of the pinhole 5 with respect to the semiconductor layer 3, whereby the pinhole 5 is filled with the material of the semiconductor layer 3, as shown in FIG. 10(d). The filled portion of the semiconductor layer 3 is initially in a fused state but it is eventually cooled, wherein the material of the filled portion is changed from an amorphous state to a polycrystalline state or the like and the photoelectric junction is broken. Thus, the filled portion functions substantially as an insulator.
Finally, as shown in FIG. 10(e), a 2,000 to 10,000 .ANG. thick aluminum layer as a back electrode 4 is laminated on the semiconductor layer 3 having the above portion filled by a vacuum evaporation technique.
The above-mentioned methods are effective in order to solve the foregoing problems relative to short circuits caused by pinholes formed in the semiconductor layer to a certain extent, but there still exist such problems as will be mentioned below, which are necessary to be solved.
In the case of the method according to Literature 1, there is a problem that it takes a long period of time in order to detect a number of pinholes present in the semiconductor layer of a large area by way of the laser beam scanning process.
There is also another problem for the method according to Literature 1. That is, as is apparent from FIGS. 10(a) and 10(b), the pinhole 6 is left as is, in any case. Such pinhole must to be filled in practice. Particularly, in the case of a semiconductor device in which a pinhole is left without being filled, water, alkali metal, or the like are apt to enter therethrough upon use. Once water or/and alkali metal, etc. enter thereinto, the semiconductor device will be quickly deteriorated. However, in order to fill up such pinhole, not only a specific technique is required but it also takes time, and because of this, the resulting product becomes unavoidably costly.
Likewise, there are problems also for the method according to Literature 2. That is, in the case of the method according to Literature 2, as is apparent from FIGS. 10(c) through 10(e), the pinhole 5 is filled by fusing the peripheries thereof, but this process is performed prior to forming the back electrode layer 4, and because of this, a pinhole which is formed at the time of forming the back electrode layer on the semiconductor layer 3 having the filled portion is unavoidably left without being filled. In addition to this, since the filling of the pinhole 5 is performed through the laser beam-irradiating process which takes time while exposing the semiconductor layer 3 to environmental atmosphere, the quality of the semiconductor 3 is apt to deteriorate during the filling process. Further in addition, powdery materials are formed when the semiconductor layer 3 is liquefied or vaporized because of heat from the laser beam irradiated thereto, wherein such powdery materials are scattered and deposited on the surface of the semiconductor layer and as a result, the device becomes defective in its characteristics.
As for the method according to Literature 2, there are disadvantages in that it takes a long period of time in order to detect a number of pinholes present at the semiconductor layer of a large area by way of the laser beam process; and it is extremely difficult to fill all such numerous pinholes uniformly by way of the laser beam fusing process.
In view of the above, in the case of the method according to Literature 2, if a desirable semiconductor device should be obtained, it will be unavoidably costly.